Grimm's Law

A law discovered by Jacob L. Grimm, the German philologist, to
show how the mute consonants interehange as corresponding words occur
in different branches of the Aryan family of languages. Thus, what is
p in Greek, Latin, or Sanskrit becomes f in Gothic, and b
or f in the Old High German; what is t in Greek, Latin,
or Sanskrit becomes th in Gothic, and d in Old High
German; etc. Thus changing p into f, and
t into th, “pater” becomes “father.”